Impedance transformers



Dec. 20, 1955 c; E oss 2,728,051

IMPEDANCE TRANSFORMERS Filed Aug. 15, 1952 2 Sheets-Sheet 1 H Hal 8 7 a 2 6 4 6 52m. i-1 firm i n V INVENTOR By CZER ROSE AT TORNEV 2,728,051 IMPEDANCE TRANSFORMERS Charles F. P. Rose, Ashury Park, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York,

Y., a corporation of New York Original application May 18, 1949, Serial No. 94,01}. Divided and this application August 15, 1952, Serial No. 304,460

2 Claims. (Cl. 333-35) transformation ratio.

A general object of the invention is to improve transformers of the above-described general type from the standpoint of simplicity, flexibility and reduction in manufacturing costs.

Another object is to efiiciently couple transmission devices of diiferent impedence characteristics without intro ducing appreciable transmission losses.

A more specific object is to match the impedance of a high metal shell member wavelengths long slidable longitudinally along the inside of the outer conductor of the connecting coaxial line. The inside of the outer shell member is correspondingly tapered at one end for /6 United States Patent 0 Patented Dec. 20, 1955 device; two cylindrical tubular sleeve members ductive material, each in drawings in which:

Fig. 1 shows a front elevation assembly view, drawn full scale, partly in cross-section and Figs. 4 to 7, inclusive, show respectively sectional views taken along the lines 4 4, 55, 66 and 77 of the assembly of Fig. 2 to illustrate various structural details of the second embodiment of the invention.

Fig. 1 shows one embodiment of the impedance transimpedance somewhat different from that of the coaxial line, and also reactance; whereas the antenna may have an impedance which is predominately resistive.

The impedance transformer of Fig. 1 includes a central core member 3 and an outer shell member 4 both made from a conductive material, such'as brass. The core member 3, which is of frusto-conical shape and has a length which is A of a wavelength (3M4) of the wave to be transmitted from magnetron M to antenna A, is mounted with its thicker end nearest the lower impedance, represented in this case by the magnetron M, coaxially within the outer conductor 2 of said coaxial line, by means of a central opening of constant diameter along its longitudinal axis (axis of member 3) on the inner coaxial line conductor 1 so as to be slidable longitudinally therealong in either direction. The shell member 4, which is cylindrically shaped on the outside and has a length which is A of a wavelength (5V 4) of the wave to be transmitted, is arranged to be slidable longitudinally in either direction along the inner surface of the outer coaxial line conductor 2. The inside of the shell member 4 is of circular cross-section and is tapered at one end nearest the magnetron M for /4 of a Wavelength (3M4) so that for one adjusted position of the two members 3 and 4 with respect to each other along the coaxial line, the tapered portion of member 4 Will closely surround the frustoconical core member 3 throughout the entire length (3M4) of the latter, and is cylindrical for the remainder of its length.

As shown, the outer diameter of the frusto-conical core member 3 at its thicker end equals the inner diameter of the cylindrical portion of the outer shell member 4, and the outer diameter of the member 3. at its other end is slightly larger than the outer diameter of the inner coaxial line conductor 1. The core member 3 is recessed slightly on the inside for a given length at its thicker end, and this recessed portion is provided with one or more circumferential rows of rhosphor bronze spring fingers 5 with coin silver ends to hold the member 3 in good contact with the outer surface of the inner coaxial line conductor 1. The outer surface of the outer shell 4 at its thicker end, is similarly recessed for a substantial portion of its length, and one or more circumferential rows of phosphor bronze spring fingers 6 with coin silver ends areattached thereto to provide good electrical contact of the outer shell member 4 with the inner surface of the outer coaxial line conductor 2 at all times. Preferably, as shown, the longitudinal recess in the oter surface of shell member 4 extends to the center thereof to provide adequate contact at the current maximum point and thereby reduce sparking. The other ends of the two slidable members 3 and 4 contain longitudinal slits (not shown) to provide enough flexibility of these members to maintain the associated spring fingers in close contact with the outer-surface of the inner coaxial line conductor 1 and the inner surface of the outer coaxial line conductor 2, respectively, when the two members 3 and 4 are moved longitudinally in either direction. A long slot 7 is cut inthe side of the outer coaxial line conductor 2, at an intermediate point of the coaxial line, and transverse apertures 8 and 9 are provided, as shown in the tapered portion of the outer shell member 4 near its thinner end, and in the inner core member 3 near its thicker end, respectively.

The manner of adjustment of the impedance transformer of Fig. 1 so that it will match the impedances of the magnetron M and the antenna A to a high degree of accuracy will now be described.

With the members 3 and 4 initially positioned so that the apertures 9 and 8, respectively, therein are aligned vertically with each other and an intermediate point along the slot 7 in the outer coaxial line conductor 2, the operator will insert the end of a rod 10 of insulating material, such as polystyrene, through slot 7 and both apertures 8 and 9. The operator will then push the rod 10 to the left or right against the sides of apertures 8 and 9 so as i to slide both members 3 and 4 as a unit longitudinally along the associated inner and outer coaxial line conductors 1 and 2, respectively, towards or away from the magnetron M until these members are located at the optimum distance from the magnetron at which the reactance component of the latter is effectively neutralized. When this point is found, the inner core member 3 is maintained stationary in its adjusted position, and the end of the rod 10 is withdrawn from aperture 9 in the core member 3 but is maintained within the aperture 8 in the outer shell member 4. The operator will then push the rod to the left or right along the slot 7 so that it will bear against the side of aperture 8 in shell member 4 causing that member to slide longitudinally along the inner surface of the outer coaxial line conductor 2 until its longitudinal proximity with respect to the stationary core member 3 is properly adjusted to match or equalize the resistance components of magnetron M and the antenna A.

The optimum adjustment of the position of the core member 3 and shell member 4 of the impedance transformer of Fig. 1 may be determined by trial and error measurements of the standing wave ratio (ratio of maximum to minimum voltage) within the coaxial line for different adjustments of the members. When the observed standing wave ratio is a minimum the impedances of magnetron M and its antenna A are properly matched. Such measurements may be made with a standing Wave detector of any of the well-known types, for example, ploying a movable probe within the coaxial line, coupled through a coaxial jack in the side of the line to a measuring circuit employing a crystal detector, an amplifier and an oscilloscope in tandem.

The characteristic impedance of" any portion of the impedance transformer shown in Fig. 1 may be determined from the following conventional formula applicable to an air-dielectric coaxial line:

log

where Zo=the characteristic impedance of the coaxial line,

D -the inner diameter of the outer conductor of the coaxial line, and

d=the outer diameter of the inner conductor of the coaxial line.

An experimental impedance transformer such as shown in Fig. l was built and tested in the output circuit of a magnetron working at a wavelength of approximately 10 centimeters into a coaxial line having an inner concentric conductor of 0.625-inch diameter and an outer concentric conductor of 1.527-inches diameter. The largest diameter X of the frusto-conical member 3, which equals the inside diameter of the cylindrical portion of the outer shell member 4, was determined for this transformer as follows:

X l.527 .625 X X =.955

X:.977 The characteristic impedance of the coaxial line is 1.527 X =l38log .625 =03.5w

The impedance Z; at the point of the largest diameter of the core member 3 when the members 3 and 4 are positioned as shown in Fig. 1 is .977 1;"27 21:13:; log =138log 9 =26.8w The transformation ratio N of the transformer equals Z 53.5 Z1 26.8

In the test referred to above, a position was found for adjustment of the inner core member 3 and outer shell member 4 of the experimental transformer which presented an optimum impedance to the magnetron and, therefore, allowed maximum power to be supplied to the load (antenna) beyond the transformer. The test showed of the inner core member 3 a /2 wavecludes a hollow housing 11, which is shown as being of square cross-section but may be of circular, rectangular or other suitable cross-section, connecting the two sections L1 and L2 of coaxial line having inner and outer concentric conductors 1 and 2, respectively, leading to the magnetron M and the antenna A, respectively. The inside 11a of housing 11, which is cylindrically shaped and may be of the same circular cross-section ,as the outer conductor 2 of the coaxial line forms a connecting link between the portions of the outer coaxial line conductor 2 in line sections L1 and L2 extending through the end plates 12 and 13, respectively, of that housing as indicated in Fig. 3. Two cylindrical tubular or sleeve members 14 and 15 of a conductive material, such as brass, each having a length of A1 of a wavelength (A/ 4) of the operating frequency of the magnetron M and inner and outer diameters determined by the desired characteristic impedance of the transformer, are mounted coaxially within the housing 11 in cascade relation with each other in the manner to be described.

Two metal rods 16 and 17 extend in parallel with each other and the longitudinal axis of the housing 11 between and are fixedly attached to the block 18 attached to and projecting vertically above the top of housing 11 at one end thereof and a fixed bracket 19 of inverted-U shape g which is attached to and extends above the top of housing 11 at its other end. Two movable brackets 20 and 21 also of inverted-U shape have their upper 16 and 17 and their two vertical legs 26b and 20c and 21b and 21c, respectively, of suitable length extending downwardly from the transverse arms on opposite sides of the outside of housing 11. Four 26 and 27 of dielectric material, such Two of the dielectric members 22 and 23, which have their outer ends respectively attached to the bottom of a different one of the two vertical legs Ztlb and 26c spectively, in opposite sides of the housing 11 into the interior thereof, Where their other ends are respectively attached at diametrically opposite points to the circumference of the tubular member 15 so as to support that member in coaxial relation with the housing 11 when it rods 16 and 17.

The longitudinal movement of 20 and 21 along the rods the movable brackets 16 and 17 is controlled by a of the shaft of gear 29 and to one end of the shaft of gear 30, to cause longitudinal movement of the rack 28 in one direction or the other.

A metal strap 33 having a longitudinal slot 34 therein, has one end affixed to the top of the transverse arm 20::

end bracket 19, 35 having a wing nut top, inserted through the slot 34 in strap 33, is adapted to be screwed into a threaded hole in the top of the transverse arm 21a of movable bracket 21 to clamp the latter to strap 33 at a desired point along slot 34; and a similar set top, inserted through the slot 34 in strap be screwed into a threaded verse arm 19a of the fixed bracket 19 to clamp the latter to strap 33 at a desired point along slot 34.

The manner of adjustment of the impedance trans- Will then loosen set screw 35' to unclamp movable bracket 21 from strap 33 and will rotate the knob 31 attached to the shaft of pinion gear 29 to the right or left to cause a corresponding right or left rotation of gear 29. This will move the movable bracket 21, because the latter is now unclamped from strap 33 with set screw 35 loosened, and thus the tubular member 15 within housing 11, to the right or left until the longitudinal proximity of the tubular member 15 with respect to the stationary tubular member 14 is properly adjusted to match or equalize the resistance components of the magnetron M and antenna A. The

' set screw 35 will then be tightened so as to clamp the movable bracket 21 to the strap 33 and maintain the relative longitudinal position of the tub lar members 14 and 15 within the housing 11 at theoptimum point found.

The optimum adjustment of the positions of the tubular members 14 and 15 of the impedance transformer of Figs. 2 to 7, as in the case of the impedance transformer of Fig. 1, may be determined by a series of trial and error measurements of the standing wave ratios within the housing 11 for different adjustments of the members, using a standing wave detector.

The mean diameter of each tubular member 14 and 15 is made equal to the geometrical mean of the diameters of the inner and outer concentric conductors 1 and 2 of the coaxial line. This was done to accommodate the greatest permissible irregularity members 14 and 15 in assembly of the transformer. The other dimensions of the members 14 and 15 were made such as to satisfy the following well-known equations Zn gio gm a b b d ZB=138 log -i-log '-c where Zn=the characteristic impedance of tubular member 14 or 15; a=outer diameter of inner line conductor 1; b=inner diameter of tubular member 14 or 15', c=outer diameter or tubular member 14 or 15', d=inner diameter of outer line conductor 2.

Z y Z Z where Z4=sending impedance at input of transformer; and Zc=terminating impedance at output of transformer.

Furthermore, a A wavelength (M4) beyond the receiver end of the transformer, an impedance Zn is seen, where Z4 4 Z1 =Z 01' 2-, In the practical embodiment of the impedance transformer of the invention shown in Figs. 2 to 7, which was constructed for use with a coaxial line having an inner concentric conductor of 0.625 inch diameter and an outer concentric conductor of 1.527 inches diameter, the inside diameter and outside diameter of each tubular member were 0.914 and 1.039 inches, respectively, and the design of the elements determining the range of longitudinal adjustment of the members 14 and 15 was such as to allow a minimum longitudinal spacing between them of not less than a A wavelength and a maximum longitudinal spacing between them at least a /2 wavelength beyond that point.

A unique feature of the impedance transformer of Figs. 2 to 7 is that the mounting of the tubular members 14 and 15 within the housing 11 is such so that they do not make metallic contact with the inner wall of the housing during adjustment of the position of these members thereby eliminating the sparking difficulties involved in transformers utilizing sliding of the variable elements along metal surfaces.

Various modifications of the impedance transformer arrangements illustrated and described which are within the spirit and scope of the invention skilled in the art.

What is claimed is:

l. A transformer for matching the impedance of a generator of oscillations of a given high frequency to that of a load device of a difierent impedance characteristic connected thereto by a coaxial transmission line having inner and outer concentric conductors, comprising a hollow housing of conductive material cylindrically shaped on the inside, inserted in said line, the interior of said housing in alignment of the two being electrically coupled through opposite ends thereof to sections of said line connected to said generator and said load device, respectively, two cylindrical sleeve members of conductive material mounted coaxially within said housing in cascade relation with each other, each of said members having an electrical length equal to A of a wavelength of said given frequency and selected outer and inner. dimensions proportioned with respect to the inner diameter of the outer concentric line conductor and the outer diameter of the inner concentric line conductor, respectively, to provide a predetermined characteristic impedance for the members, means for establishing said members in fixed longitudinal relationship with respect to each other, means for simultaneously moving both of said members in said fixed longitudinal relationship in either direction along the axis of said housing Without metallic contact with any part thereof, to a position at which the reactance components of the impedances of said generatorand said load devices are effectively neutralized and means for maintaining one of said members in that position while separately adjusting the longitudinal proximity of the other member with respect thereto'to effectively match the resistance components of the impedances of said generator and said load device, said members being adapted for longitudinal positioning within said housing through the intermediary of two brackets affixed to and extending above said housing at opposite ends thereof, a plurality of rods extending in parallel with the longitudinal axis of said housing between and affixed to said brackets, two movable brackets each having a transverse arm slidably mounted on said rods and two vertical legs extending downwardly from said arm on opposite sides of said housing, a centrally-located longitudinal slot in each side of said housing and four dielectric members respectively having one end thereof aflixed to the bottom of a different one of the vertical legs of said two movable brackets and extending therefrom transversely through one of said slots to the interior of said housing, the two of said dielectric members which have their said one ends respectively affixed to the bottom of one of two of the vertical legs of one of said movable brackets, having their other ends respectively affixed to one of said sleeve members at diametrically opposite points on the circumference thereof, and the other two dielectric members which have their said one ends respectively affixed to the bottoms of a different one of the two vertical legs of the other of said movable brackets, having their other ends respectively affixed to the other of said sleeve members at diametrically oppositepoints on the circumference thereof.

2. A transformer for matching the impedance of a generator of oscillations of a given high frequency to that of a load device of a different impedance characteristic connected thereto by a coaxial transmission line having inner and outer concentric conductors, comprising a hollow housing of conductive material cylindrically shaped on the inside, inserted in said line, the interior of said housing being electrically coupled through opposite ends thereof to sections of said line connected to said generator and said load. device, respectively, two cylindrical sleeve members of conductive material mounted coaxially within said housing in cascade relation with each other, each of said members having an electrical length equal to /4 of a wavelength of said given frequency and selected outer and inner dimensions proportioned with respect to the inner diameter of the outer concentric line conductor and the outer diameter of the inner concentric line conductor, respectively, to provide a predetermined characteristic impedance for the member, means for establishing said members in fixed longitudinal relationship with respect to each other, means for simultaneously moving both of said members in said fixed longitudinal relationship. in either direction along the axis of said housing without metallic contact with any part thereof, to a position at which the reactance components of the impedances of said generator and said load devices are effectively neutralized and means for housing, and separate means for respectively locking a maintaining one of said members in that position while difierent one of said movable brackets in fixed longiseparately adjusting the longitudinal proximity of the tudinal position With respect to the ends of said housother member With respect thereto to efiectively match ing or with respect to the other movable bracket. the resistance components of the impedances of said gen- 5 erator and said load device, the mounting of said sleeve Refermces Cited in the file of this Patent members including a pair of brackets afiixed to and pro- U T D AT AT N jecting above said housing at opposite ends thereof, one or more rods extending in parallel with the longitudinal 1 1942 omas June 26, 1945. axis of sa1d housing between sa1d fixed brackets and a 10 2 408 7 4 Espley Oct 8 1946 pair of movable brackets slidable alon said rod or rods Latimer 1950 for respectively suspending a dlfiFerent one of sa1d sleeve members within said housing in coaxial relation there- FOREIGN E T with and said longitudinal position adjusting means com- 434 225 Italy APR 1 94 prising a rack and pinion gearing arrangement, associated 15 with said movable brackets, manually operable control OTHER REFERENCES knobs for respectively controlling operation of said gear- Ragan: Microwave Transmission Circuits, vol. 9,

mg to move a difierent one of said movable brackets lon- M. I. T. Radiation Laboratory Series, Copyright May gitudinally along said rod or rods, and thus a difi erent 21, 1948, pages 466-472, and 494-498. (Copy in Patone of said sleeve members longitudinally within said 20 cut Ofiice Library.) 

